1. Field of the Invention
The invention relates generally to the cooling of solid particles from applications that are operating at relatively high temperatures and pressures. It is particularly related to cooling high temperature ash from a coal gasifier operating in a temperature range of from approximately 1500° F. to 2200° F., and a pressure range of from approximately 30 to 1000 psia.
2. Description of Related Art
The cooling of hot solids from a gasifier or reactor that is operating in the temperature range of from approximately 1500° F. to 2200° F., and a pressure range of from approximately 30 to 1000 psia presents numerous challenges, none of which have all be overcome by conventional systems.
A first challenge is supporting the heat exchanger tubes that will exchange the heat from the solids to a cooling media. The difficulty in this issue is that the support has to be anchored to the outside wall, penetrating through layers of refractories that are necessary to resist the erosion due to movement of solid particles (in the mass mean diameter size range of from approximately 50 microns to 400 microns), and to insulate the wall from overheating.
Aeration gas to facilitate the movement of hot particles and flow of particles past the cooling surface induces vibration in the cooling tubes and support. The vibration of the support can damage the refractory, and cause the vessel wall to locally overheat. Heat conduction through the support can also overheat the vessel walls, damaging and deforming the vessel. This is a serious concern as the vessel forms the pressure boundary.
A second challenge in the development of a high pressure, high temperature heat exchanger is to achieve appropriate control of the solids flow to the heat exchanger without interfering with the operation of the gasifier or reactor from which the solids are being withdrawn and/or to which the cooled solids are being returned. Also, for the circulating fluidized bed gasifier, when the solids are withdrawn from the standpipe, the aeration gas cannot be returned to the standpipe or the gasifier due to pressure restrictions. Return of aeration gas through the take-off point impedes solids flow to the cooler. Handling of the vent gas is difficult as the gas entrains fines at high process temperatures. Under these circumstances, a challenge becomes how to vent the aeration gas and a portion of the gas entrained by the solids.
A third challenge is optimizing the design of the cooler so that the solids in the cooler, when they come in contact with the heat transfer surfaces, have a temperature range of from approximately 800° F. to 1000° F. Such a consideration improves reliability and durability of the cooler heat transfer surfaces, and facilitates the use of low cost steel for the cooling surface. Although the solids at the inlet of the cooler have a temperature range of from approximately 1500° F. to 2200° F. as they are withdrawn from the gasifier, a robust cooler design necessitates that the solids contacting the heat transfer area have a temperature less than approximately 1000° F. Known exchanger designs have one or two tubesheets supporting the heat exchanger tubes. The tubesheet diameter tends to be large in commercial coolers. It is prudent to design the cooler without exposing the tubesheet to hot solids.
A fourth challenge in cooler equipment design involves appropriate handling of foreign and extraneous materials that originate from or pass through the gasifier. Foreign and extraneous materials in the process result from, for example, contaminated feed, chipped refractory, broken gasifier internals and clinkers and slag formed during the process due to variability in feed fuel (coal, for example) or improper operation. These materials are generally oversized and need to be removed from the process before reaching the heat exchanger surfaces to limit or prevent blockages in the flow path of hot solids.
Conventional systems that cool hot solids from a reactor mainly fall into two application areas: cooling hot solids (catalyst particles) from the fluid catalytic cracking (FCC) process, and from the circulating fluidized bed (CFB combustors) boilers.
In the FCC area, examples include U.S. Pat. No. 4,424,192 to Lomas et al., U.S. Pat. No. 4,425,301 to Vickers et al., U.S. Pat. No. 4,822,761 to Walters et al., and U.S. Pat. No. 5,209,287 to Johnson et al. These teachings are applicable to relatively low pressure processes, as FCC processes normally operate below approximately 50 psi. Among these examples, Johnson et al. discloses using a screen to prevent extraneous materials from entering and interfering with the cooler operation. As one of skill in the art appreciates, however, with gasifier operation, it is highly desirable to discharge extraneous materials from the gasifier, as accumulation of extraneous materials in the gasifier can cause various operating issues including formation of clinkers in the gasifier.
The FCC designs include the hot solids entering the cooler from the top, and the cooled solids exiting from the bottom or side of the vessel near the bottom. Thus, these references disclose systems that require the gas velocity be sufficiently high to fully fluidize the bed particles in order to guarantee that the bed reaches uniform temperature. This is not an issue in the FCC process because the catalyst particle size is relatively uniform, and it is relatively easy to achieve uniform fluidization within a narrow range of the gas velocity.
As one of skill in the art appreciates, the situation is quite different in gasification and combustion processes where the particle size can be in the range of approximately 30 microns to 10,000 microns, and the complete fluidization velocity in the cooler has to be near the minimum fluidization velocity of the largest particle size in the cooler. For 10,000 micron particles, the minimum fluidization velocity can be as high as approximately 10 ft/s, and operating at such high velocities requires large amounts of gas flow through the cooler. It is difficult to return such a large amount of gas flow through the cooler to the gasifier or combustor without interfering with its normal operation.
Another issue with the FCC references is that if the extraneous materials, which are common in gasification and combustion processes, pass through the cooling bundle, they can segregate and accumulate in the bottom of the cooler, eventually interfering with the normal operation of the cooler, since the FCC design has the solids downward flow and side withdrawal near the bottom. It is difficult to apply these teachings to cool the gasifier solids with a broad particle size distribution such as from a fluidized bed or a circulating fluidized bed gasifier.
In the CFB area, examples include U.S. Pat. Nos. 5,510,085 and 5,463,968 to Abdulally, U.S. Pat. No. 5,184,671 to Allison et al., and U.S. Pat. No. 7,194,983 to Kokko. In these teachings, both the solids and the fluidization gas return to the combustor to maintain combustion temperature. As these references disclose in-process coolers, the outside surface of the cooling tubes are essentially in contact with the solids, which solids have temperatures near the operating combustor temperature of approximately 1600° F. Although such operating temperatures make it necessary to use expensive alloy materials for the heat exchanger, the overall environment is tolerable for most alloy engineering materials. As one of skill in the art appreciates, however, with gasifier operation, the operating temperature can reach as high as approximately 2000° F.; thus, the materials selection can be a challenge or the materials cost will be prohibitive when the hot solids at such high temperatures directly contact the heat transfer surface.
Further, except for Kokko, the other CFB examples cited disregard the detrimental effects of extraneous materials entering the heat exchanger. Kokko recognizes the importance of avoiding solids by-passing some of the heat transfer surface, and devises a way to ensure that solids will flow through the entire heat transfer surface. However, in Kokko's design, solids have to make turns in three chambers that naturally makes the flow of solids more complex and more difficult to handle extraneous materials.
U.S. Pat. No. 7,464,669 to Maryamchik et al. discloses an ash cooler with two chambers-one for discharge of coarser ash and another for finer particles. However, the large particle ash chamber does not have a cooling surface, and therefore the ash withdrawn from the chamber is essentially the same temperature as that in the combustor. It is also difficult to achieve good separation of coarser and finer particles in a fluidized bed. In Maryamchik et al., the fluidization gas returns back to the combustor, a practice which may not be feasible for some applications.
Further, Maryamchik et al. discloses that the tube bundles for cooling the solids penetrate through the refractory walls. For CFB boilers, this practice is not a major issue because the combustor is essentially operated near atmospheric pressure. Even if there is damage to the refractory, it would not lead to catastrophic vessel wall failure due to this low pressure operation. As one of skill in the art appreciates, however, with gasifier operation at high pressure, the cooling surface penetrating the wall can become a serious safety issue, and no known solution exists, other than to avoid it altogether. Further, the cooling surface in the heat exchanger will still be in contact with finer particles essentially at the same high temperature of approximately 1600° F. as that in the combustor, necessitating the use of expensive engineering alloy materials for heat transfer surfaces.
US Patent Publication No. 2009/0300986 to Liu discloses cooling gasification ash from a fluidized bed gasifier. In Liu, the extraneous materials are screened out at the inlet to the cooler and collected in a separate vessel. In this arrangement, substantial recycle gas must be used to purge the small particles from extraneous materials. Substantial recycle gas must be used to prevent small particles from entering the solids cooler, and also for continuously purging the screen to ensure that it remains plug-free. This combination of large purge gas flows and handling the high temperature particles increases the material, fabrication and operation costs.
In Liu, all the purge and fluidization gas flows back to the gasifier, impeding operations if the flow is excessive. Further, the cooling surface of Liu penetrates the refractory and vessel walls of the cooler causing potential difficulties with cooler wall design even with gasifier operating pressures being less than approximately 50 psi. During operation, the cooling surface contacts solid particles that are near the high gasifier operating temperatures, which leads to challenging and expensive design.
What are needed are cost effective and reliable solutions to cool the high temperature, high pressure ash from a gasifier, and other similar applications. It is to such systems and methods that that present invention is primarily directed. The present invention overcomes the various challenges discussed previously, and provides a system for cooling high temperature ash from a coal gasifier operating in a temperature range of from approximately 1500° F. to 2200° F., and a pressure range of from approximately 30 to 1000 psia.